A longstanding issue in environmental health is the need to understand the role the environment plays in human brain development. The brain of the neonate is particularly susceptible to disruption of the sensory environment, which can have profound effects on its physiology and morphology. Such susceptibility of the developing brain to environmental influence by sensory manipulation or to environmental toxicants is particularly pronounced during defined critical periods of postnatal life. On the one hand, this susceptibility makes the developing brain particularly vulnerable to toxic insults. On the other hand, the plasticity of the connections between neurons, or synapses, is critical for refining brain circuitry during postnatal development. Similar mechanisms for changing synapses are likely to serve the basis for learning in the adult. Our primary interest, therefore, has been to determine the molecular basis of long-lasting synaptic plasticity. Toward our goal of learning how neuronal activity can induce lasting modifications in neurons, we use a diverse collection of molecular, biochemical, electrophysiological, and imaging techniques. We primarily use the hippocampal slice preparation using neonate and adult rats and mice. The relatively simple laminar structure of the hippocampus, which itself plays an important role in learning and memory, allows electrophysiological studies to be performed easily. To measure synaptic responses, we use techniques that include whole-cell patch clamp recordings and field potential recordings from acutely prepared hippocampal slices maintained in vitro. Slice-cultures grown on multielectrode arrays allow for extracellular stimulating and recording during two-photon confocal fluorescent microscopy. To determine how transcription is regulated by neuronal activity, we use molecular and biochemical methods with acutely dissociated hippocampal and cortical neuronal cell cultures, which can be stimulated pharmacologically to mimic long-term potentiation (LTP) and long-term depression (LTD). To understand how synaptic changes persist for up to a lifetime, we study how neuronal activity regulates gene transcription to consolidate synaptic changes. Evidence suggests that the long-term changes in synaptic efficacy require expression of new RNA and toward that end, we have focused on the regulation of gene transcription by neuronal action potentials. Previously, we have shown that action potentials are necessary and sufficient to initiate protein kinase activation and subsequent gene transcription in neurons. These findings have important implications for the interpretation of experiments using NMDA receptor inhibitors to conclude that signals to the nucleus come from the synapse. In an effort to provide a mechanism for how genes might be rapidly transcribed without necessarily involving biochemical signaling from synapses, we have investigated the transcriptional readiness of genes rapidly regulated by neuronal activity. We found that several immediate early genes (IEGs) such as arc/arg3.1 are poised for near-instantaneous transcription by the stalling of RNA Polymerase II (Pol II) just downstream of the transcription start site in rat neurons. RNAi-depletion of Negative Elongation Factor, a mediator of Pol II stalling, reduced the Pol II occupancy of the arc promoter and compromised the rapid induction of arc and other IEGs. In contrast, reduction of Pol II stalling did not prevent transcription of IEGs that are expressed later and largely lack promoter proximal Pol II stalling. Together, our data strongly indicate that rapid induction of neuronal IEGs requires poised Pol II and suggest a role for this mechanism in a wide variety of transcription-dependent processes. These results also support our idea that action potentials can be the critical trigger for transcription of some genes required for the consolidation of synaptic plasticity. One approach that we have taken to gain insight into the mechanisms of synaptic plasticity has been by comparing highly plastic brain areas, such as the CA1 area of hippocampus, with less plastic areas, such as layer 4 of the cerebral cortex. From the expression pattern of some genes, we predicted and found that one area of the hippocampus, the CA2, would share with layer 4 a resistance to synaptic plasticity including LTP and LTD, even though synaptic responses in CA2 were very similar to those in the neighboring CA1 and CA3 areas. Because LTP requires postsynaptic calcium for its induction, a first step in assessing how CA2 differs from its neighboring subfields was to test whether comparable levels of free calcium could be achieved in CA2 neurons in response to action potential generation. Using 2-photon laser scanning microscopy and whole-cell recordings of hippocampal neurons in slices, we found that dendritic spines in CA2 have very different calcium dynamics from spines in CA1 and CA3. Both calcium buffering capacity and rates of calcium extrusion were higher in CA2 spines when compared with those in the neighboring regions. When calcium extrusion was disrupted by inhibition of the plasma membrane calcium ATPase, LTP was restored. Thus calcium handling is one way that plasticity is negatively modulated in CA2. In collaboration with John Hepler at Emory University, we discovered a second negative modulator of synaptic plasticity in CA2: RGS14, which is highly enriched there. We found that loss of RGS14 results in mice that exhibit robust nascent LTP in CA2 neurons, with no effect on adjacent CA1 neurons where RGS14 is not present. We also found that these mice exhibit enhanced spatial learning and novel object memory previously associated with the hippocampus. These findings implicate, for the first time, RGS14 and the CA2 in synaptic plasticity relating to hippocampal learning and memory. Using information we learn from CA2, we aim to determine the nature of the developmental down-regulation of synaptic plasticity in the form of critical periods and how plasticity is modulated in different brain areas. The CA2 region, incidentally, has been noted for its resistance to disease and damage from trauma, ischemia, and stroke. Because CA2 and its surrounding regions are anatomically very similar, these findings may therefore lead to identification of critical molecular components in the pathways leading to not only synaptic plasticity, but also neuronal damage and death. Our longer term goal is to determine how neuronal activity leads to synapse elimination (pruning). We have now developed a technique by which activity-dependent synapse elimination during critical periods can be studied in live tissue. We found that electrical stimulation that results in LTD is accompanied by loss of synaptic connections and that smaller synaptic contacts were most likely to be eliminated. This method will allow us to test our ideas on the molecular mechanisms allowing LTD to lead to synaptic loss. By understanding the molecular and cellular mechanisms of synaptic plasticity during development, we may begin to understand how exposure to environmental toxicants during development can have life-long consequences on cognition and susceptibility to diseases such as autism, schizophrenia, and Alzheimers disease.